[pmden]

I believe boot chess also does not implement two of those three, which makes it a fine comparison. Promotion is something I expect most people know, castling is something they might know, but en passant is hardly common. I wouldn't be at all surprised if the number of people who play chess without knowing that rule exceed those who play by FIDE rules.

Whatever the case, it is still a variant of chess, so I see no problem calling it chess.

[umanwizard]

Promotion shouldn't be compared to castling/en-passant here.

A game without castling or en passant would be similar enough in spirit that we could call it a "chess variant". Removing promotion, on the other hand, would result in a wildly different game.

I actually suspect it'd be impossible to win against any non-beginners, making it more similar to tic-tac-toe than to chess.

[spacehome]

En passant comes up a lot more often when you don't know the rule.